Function mdb_env_open

pub unsafe extern "C" fn mdb_env_open(
    env: *mut MDB_env,
    path: *const c_char,
    flags: c_uint,
    mode: mdb_mode_t,
) -> c_int

Open an environment handle.

If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.

Arguments

• env (direction in) - An environment handle returned by #mdb_env_create()
• path (direction in) - The directory in which the database files reside. This directory must already exist and be writable.
• flags (direction in) - Special options for this environment. This parameter must be set to 0 or by bitwise OR’ing together one or more of the values described here. Flags set by mdb_env_set_flags() are also used.

• #MDB_FIXEDMAP use a fixed address for the mmap region. This flag must be specified when creating the environment, and is stored persistently in the environment. If successful, the memory map will always reside at the same virtual address and pointers used to reference data items in the database will be constant across multiple invocations. This option may not always work, depending on how the operating system has allocated memory to shared libraries and other uses. The feature is highly experimental.
• #MDB_NOSUBDIR By default, LMDB creates its environment in a directory whose pathname is given in **path,** and creates its data and lock files under that directory. With this option, **path** is used as-is for the database main data file. The database lock file is the **path** with "-lock" appended.
• #MDB_RDONLY Open the environment in read-only mode. No write operations will be allowed. LMDB will still modify the lock file - except on read-only filesystems, where LMDB does not use locks.
• #MDB_WRITEMAP Use a writeable memory map unless MDB_RDONLY is set. This uses fewer mallocs but loses protection from application bugs like wild pointer writes and other bad updates into the database. This may be slightly faster for DBs that fit entirely in RAM, but is slower for DBs larger than RAM. Incompatible with nested transactions. Do not mix processes with and without MDB_WRITEMAP on the same environment. This can defeat durability (#mdb_env_sync etc).
• #MDB_NOMETASYNC Flush system buffers to disk only once per transaction, omit the metadata flush. Defer that until the system flushes files to disk, or next non-MDB_RDONLY commit or #mdb_env_sync(). This optimization maintains database integrity, but a system crash may undo the last committed transaction. I.e. it preserves the ACI (atomicity, consistency, isolation) but not D (durability) database property. This flag may be changed at any time using #mdb_env_set_flags().
• #MDB_NOSYNC Don't flush system buffers to disk when committing a transaction. This optimization means a system crash can corrupt the database or lose the last transactions if buffers are not yet flushed to disk. The risk is governed by how often the system flushes dirty buffers to disk and how often #mdb_env_sync() is called. However, if the filesystem preserves write order and the #MDB_WRITEMAP flag is not used, transactions exhibit ACI (atomicity, consistency, isolation) properties and only lose D (durability). I.e. database integrity is maintained, but a system crash may undo the final transactions. Note that (#MDB_NOSYNC | #MDB_WRITEMAP) leaves the system with no hint for when to write transactions to disk, unless #mdb_env_sync() is called. (#MDB_MAPASYNC | #MDB_WRITEMAP) may be preferable. This flag may be changed at any time using #mdb_env_set_flags().
• #MDB_MAPASYNC When using #MDB_WRITEMAP, use asynchronous flushes to disk. As with #MDB_NOSYNC, a system crash can then corrupt the database or lose the last transactions. Calling #mdb_env_sync() ensures on-disk database integrity until next commit. This flag may be changed at any time using #mdb_env_set_flags().
• #MDB_NOTLS Don't use Thread-Local Storage. Tie reader locktable slots to #MDB_txn objects instead of to threads. I.e. #mdb_txn_reset() keeps the slot reserved for the #MDB_txn object. A thread may use parallel read-only transactions. A read-only transaction may span threads if the user synchronizes its use. Applications that multiplex many user threads over individual OS threads need this option. Such an application must also serialize the write transactions in an OS thread, since LMDB's write locking is unaware of the user threads.
• #MDB_NOLOCK Don't do any locking. If concurrent access is anticipated, the caller must manage all concurrency itself. For proper operation the caller must enforce single-writer semantics, and must ensure that no readers are using old transactions while a writer is active. The simplest approach is to use an exclusive lock so that no readers may be active at all when a writer begins.
• #MDB_NORDAHEAD Turn off readahead. Most operating systems perform readahead on read requests by default. This option turns it off if the OS supports it. Turning it off may help random read performance when the DB is larger than RAM and system RAM is full. The option is not implemented on Windows.
• #MDB_NOMEMINIT Don't initialize malloc'd memory before writing to unused spaces in the data file. By default, memory for pages written to the data file is obtained using malloc. While these pages may be reused in subsequent transactions, freshly malloc'd pages will be initialized to zeroes before use. This avoids persisting leftover data from other code (that used the heap and subsequently freed the memory) into the data file. Note that many other system libraries may allocate and free memory from the heap for arbitrary uses. E.g., stdio may use the heap for file I/O buffers. This initialization step has a modest performance cost so some applications may want to disable it using this flag. This option can be a problem for applications which handle sensitive data like passwords, and it makes memory checkers like Valgrind noisy. This flag is not needed with #MDB_WRITEMAP, which writes directly to the mmap instead of using malloc for pages. The initialization is also skipped if #MDB_RESERVE is used; the caller is expected to overwrite all of the memory that was reserved in that case. This flag may be changed at any time using #mdb_env_set_flags().
• #MDB_PREVSNAPSHOT Open the environment with the previous snapshot rather than the latest one. This loses the latest transaction, but may help work around some types of corruption. If opened with write access, this must be the only process using the environment. This flag is automatically reset after a write transaction is successfully committed.

* `mode` (direction in) - The UNIX permissions to set on created files and semaphores. This parameter is ignored on Windows. # Returns

A non-zero error value on failure and 0 on success. Some possible errors are:

• #MDB_VERSION_MISMATCH - the version of the LMDB library doesn't match the version that created the database environment.
• #MDB_INVALID - the environment file headers are corrupted.
• ENOENT - the directory specified by the path parameter doesn't exist.
• EACCES - the user didn't have permission to access the environment files.
• EAGAIN - the environment was locked by another process.